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VOLCANO, Hawaii – Mauna Loa is stirring after an 18-year pause, and an eruption could be devastating to the neighborhoods built on the giant volcano’s slopes in the intervening years, scientists said Monday. 

“There has been a substantial amount of development on what has historically been the most hazardous part of Mauna Loa — its southwest rift zone above South Point,” said Peter Cervelli, a research geophysicist at the U.S. Geological Service’s Hawaiian Volcano Observatory. 

“Though lava flows can reach Hilo on the eastern side of the island and the Gold Coast resorts of Kona in the west, flows are much more likely to inundate the subdivisions in the southwest rift zone — and possibly without much warning.” 

Mauna Loa has erupted 33 times since 1843. In spring 1984, Mauna Loa erupted for three weeks, sending a 16-mile lava flow toward Hilo. Since then, the USGS estimates that more than $2.3 billion has been invested in new construction along Mauna Loa’s slopes. 

“In some cases they’re building on lava flows that are less than 100 years old,” Cervelli said. 

Scientists from Stanford University recently joined the observatory in monitoring the 13,500-foot volcano, which began to stir on May 12. 

Recent geophysical data collected on the surface has revealed that Mauna Loa’s summit caldera has begun to swell and stretch at a rate of 2 to 2 1/2 inches a year, which can be a precursor of an eruption. 

“It has not erupted in 18 years but that is an extremely long pause,” Cervelli said. “We’re at a stage where it’s months to years, rather than days to weeks.” 

“Mauna Loa is capable of erupting huge volumes of lava in a relatively short period of time, and the flows can reach great distances,” said Paul Segall, a professor of geophysics at Stanford who has worked with USGS volcanologists in Hawaii since 1990. “It presents a more significant safety hazard than Kilauea.” 

Scientists are working to detect an eruption as early as possible to give people a chance to evacuate the populated areas. 

“Earthquakes will always precede the movement of magma to the surface,” said Cervelli. “In our experience, it’s going to be at least hours.